The present application relates to a battery pack having a secondary battery, a battery charger and a charging method for charging the battery pack.
Recently, portable electric equipments such as notebook type PCs (personal computers), cellular phones and PDAs (personal digital assistants) have been spread, and as a power source thereof, battery packs using a lithium ion secondary battery have been widely used which have advantages of high voltage, high energy density and light weight. These battery packs having different capacities and different charge rates are manufactured corresponding to the equipment to be used and the purpose. In the present specification, the term “battery pack” refers to indicate those in which a secondary battery, a circuit section for controlling charge or discharge of the secondary battery, and a communication unit for performing communication with a battery charger are integrated into a single device.
The respective battery charger for charging the battery packs have conventionally been manufactured according to the characteristics such as capacities and charge rates of the battery packs. If the battery charger adapted to the characteristics is manufactured every time a new battery charger is manufactured, the manufacturing cost will increase. Furthermore, if a battery pack having substantially the same shape and a different characteristic is connected to a battery charger which is not adapted to the battery pack, a potentially dangerous condition such as heat generation or ignition may develop.
Thus, a battery charger that can perform charging so as to match a plurality of types of battery packs has been used recently. The use of this type of battery charger eliminates the need to manufacture a new charger, and the single battery charger can charge any battery packs having, for example, different battery capacities. The charger capable of charging a plurality of types of battery packs is usually designed to be able to charge a low capacity battery pack and a low charge rate battery pack among the corresponding battery packs. Consequently, for example, the charge current passing from the charger to the battery pack during charging is designed to be low so as to match the low capacity battery pack.
As a quick charging mode of a secondary battery such as a lithium ion secondary battery, a CCCV (constant current constant voltage) charging mode as a combination of constant current charge and constant voltage charge is used. In the CCCV charging mode, as shown in FIG. 9, charging is performed at a constant-current until the battery voltage of a battery pack reaches a predetermined voltage, and after reaching the predetermined voltage, charging is performed at a constant-voltage. The charging is terminated at the point that the charging current has converged to substantially a zero ampere.
For example, in the range where the battery voltage V is 4.1V or below, the constant-current charging is performed at A=500 mA. When the battery voltage (the internal electromotive force) of the secondary battery is increased by the charging, and the battery voltage V becomes greater than 4.1V, the operation of a charge power source section is switched to the constant-voltage control, and a charge current A is gradually decreased. The battery voltage V is increased toward an output voltage (4.2V) of the power source section. Subsequently, when the charge current approaches substantially a zero, the charging is completed. As a method of detecting the completion of the charging, current detection method and ΔV detection method are known.
In general, when a quick charging is performed on a high capacity battery pack, the battery pack may be charged at a higher charge current than the charge current of a low capacity battery pack. However, when the high capacity battery pack is charged by using the conventional battery charger capable of charging a plurality of types of battery packs, a charging is performed at a low charge current for charging the low capacity battery pack which is preset to the charger. Consequently, if the high capacity battery pack is charged, a constant-current charging period becomes longer, and therefore it will take much time to charge.
That is, when the high capacity battery pack or the high charge rate battery pack is charged by the conventional battery charger, the charging may not be properly to meet the characteristic thereof, whereby a problem of taking much time to charge will arise.
In order to solve the above problem, as described in, for example, Japanese Unexamined Patent Application Publication No. 9-285026 (hereinafter referred to as “Patent Document 1”), a battery charger has been proposed which can properly charge the respective battery packs by changing the charge current in accordance with the characteristics of the battery packs. The battery charger described in the Patent Document 1 obtains, for example, the information regarding a maximum charge current and a maximum charge voltage from the respective battery packs. The battery charger is designed to change the charge current in the constant-current period based on the obtained information on the maximum charge current and the maximum charge voltage.
Specifically, as shown in FIG. 11, it is set to pass the charge current in the constant-current period of a high capacity battery pack at a higher current value than that of a low capacity battery pack. This enables the high capacity battery pack to be properly charged, thereby reducing the charging time than the charging by the conventional battery charger.
Thus, battery packs having different characteristics may be properly charged, and the charging time may be reduced by changing the charge current based on the information on the maximum charge current obtained from the respective battery packs.
Meanwhile, when a normal charge current is passed through a battery pack by performing quick charging under the condition that the battery voltage of battery pack is below a predetermined voltage, an abnormal event such as heat generation may be caused. For this reason, a charge current as small as, for example, about 100 mA to 200 mA, is usually passed through the battery pack before the quick charging, so that the initial charging is continued until the battery voltage of the battery pack reaches the predetermined voltage.
A predetermined voltage (in some cases hereinafter referred to as a “switching voltage”) is preset to the battery charger, as a voltage threshold value for switching the charging mode from an initial charging to a quick charging. The quick charging is started when the battery voltage of the battery pack reaches the switching voltage during the initial charging. The battery charger is provided with a timer to which a timeout period is set. That is, when the battery voltage of the battery pack does not reach the switching voltage within a predetermined period of time, the battery pack is judged as abnormal and the charging is discontinued, based on the timeout period.
The case of charging a high capacity battery pack by using the conventional battery is now considered. The battery charger is set to, for example, the timeout period and the switching voltage to match the conventional low capacity battery pack.
As shown in FIG. 12, the high capacity battery pack has a lower speed of voltage rise than the low capacity battery pack, and therefore it takes much time until the battery voltage of the battery pack reaches the switching voltage, and it may exceed the timeout period set to the timer of the battery charger. Accordingly, in this case, even if the high capacity battery pack is in the normal state, the battery pack may be judged as abnormal, and the charging may be discontinued.
There are also battery packs having high performance so that they can discharge up to a lower voltage than the conventional battery packs as shown in FIG. 13, by changing, for example, the material used in electrodes. The high performance battery packs can discharge up to a lower voltage than the conventional battery pack, thereby making it possible to draw more amount of discharge.
Like the abovementioned high capacity battery pack, the high capacity battery pack attained by improving the performance has a lower speed of voltage rise than the low capacity battery pack. Accordingly, when charging is performed by a battery charger whose switching voltage is set so as to match the conventional battery pack, even if the battery pack is in its normal state, a charge control processor may judge the battery pack as abnormal, and stop charging because the measured time exceeds the timeout period.
The high performance battery pack can also perform a quick charging from a lower voltage than the conventional battery pack. When the high performance battery pack is charged by the conventional battery charger whose switching voltage to allow switching from the initial charging to the quick charging is set so as to match the conventional battery pack, the battery voltage of the battery pack may require a longer charging time because the initial charging is continued even after the battery voltage of the battery pack reaches a voltage which is able to switch to a quick charging, as shown in FIG. 14.
Thus, in the conventional battery charger, the timeout period and the switching voltage related to the switching from the initial charging to the quick charging are set so as to match the low capacity battery pack, thereby making it difficult to properly charge the high capacity battery pack and the high charge rate battery pack.